The present invention relates generally to radio frequency (“RF”) and electromagnetic interference (“EMI”) protection for electronic circuitry, and in particular, to a surface mount technology (“SMT”) stripline structure used in combination with an isolation wall for routing radio frequencies between two cavities that require a high level of isolation.
Printed circuit boards (“PCB”) comprise a dielectric substrate which supports the printed wiring, including both circuit and ground traces. The detailed method of construction of printed circuit board and the materials used are well known in the art. Many PCBs require a means of signal isolation to reduce or prevent RFI and EMI between various groups or blocks of circuitry. The circuit blocks comprise both active and passive components. In accordance with conventional SMT practice, these electrical components are mounted on the top surface of the printed circuit board during the automated (“component population”) phase of the manufacturing process. As used here, the terms EMI and RFI denote RF signals unintentionally coupled, radiated, or otherwise transmitted between circuit blocks that are intended to be mutually isolated. Ideally, circuit blocks that are likely to create, or to be susceptible to, EMI in or from other nearby circuitry would be contained within a single shielded enclosure or cavity.
When a shielded transmission line is used to connect or route RF signals between physically separated shielded cavities, the shielded transmission line minimizes EMI by providing a continuous extension of the shielding surfaces of the separated cavities. The internal structure of the transmission line is selected so that its characteristic impedance within the frequency bands of interest is both well defined and controlled in order to minimize RF signal distortion and maximize the transfer of desired RF signal power. Conventional examples of shielded transmission line include stripline and coaxial cable.
The term stripline commonly denotes a structure comprising a signal conducting strip and two ground planes which extend considerably in transverse directions. The space between the ground planes is filled with a dielectric medium and the central strip is embedded in this dielectric. The ground planes are at zero RF potential relative to each other. Coaxial cable utilized as an isolated interconnecting transmission line is not compatible with the automated SMT assembly process flow and generally must be hand formed and soldered at a much greater expense.
An example of an unshielded transmission line is microstrip, comprising a single dielectric substrate with ground plane on one side and a signal conducting strip on the other face. Unlike stripline, SMT components can be attached directly to the signal conducting top layer of microstrip. Microstrip is also subject to EMI from nearby conductors because of its unshielded structure.
When coaxial cable is not used, the conventional structure of SMT assemblies requiring a shielded transmission line interconnection is a multilayer PCB incorporating at least one stripline structure as described above (ground layer, dielectric, signal conducting strip, dielectric, ground layer) and optional layers for routing other signals. The performance constraints imposed by this solution include:
1) The multilayer PCB structure exhibits increased thermal resistance from the SMT component side to the reverse side which is commonly attached to a heatsink.
2) In practice, the two ground planes are conventionally connected together along two paths parallel to the entire length of the central conducting strip, in order to minimize the difference of potential between the ground planes, and thereby minimize coupling of RF signals between the stripline structure and adjacent circuitry. The ground plane interconnection is normally accomplished with conductive through-holes, thereby adding significant cost to the final PCB assembly.
3) PCB material handling and processing costs are much lower when the unpopulated PCB consists of only a back conductive layer, one dielectric layer, and a top SMT compatible layer. Converting the entire PCB to a multilayer PCB structure when only a small number of shielded interconnections are required adds unnecessarily to the final assembly weight, size and cost.
4) Specialized circuit functions including RF power amplifiers are conventionally fabricated on PCB materials, including hard or brittle ceramic substrates, that are not compatible with multilayer PCB fabrication techniques, thereby precluding the inclusion of stripline as an inherent part of the unpopulated PCB.
Those having skill in the art would understand the desirability of having a radio frequency interconnection that has high isolation, without and does not need extensive hand assembly to produce. This type of radio frequency interconnection would necessarily provide sufficient isolation, and allow surface mount technology to be utilized, thus allowing the cost efficient manufacturing of high frequency circuit assemblies to be achieved.